Lysosomes 'loaded' with supra-normal levels of free L-cystine can be isolated from intact human leucocytes which have been exposed to the dimethyl ester of the amino acid. Such loaded preparations were used to study the rate of egress of cystine from lysosomes derived from leucocytes of normal human subjects and from patients with cystinosis, an inherited disorder characterized by excessive intralysosomal accumulations of this amino acid. It has been found that the rate of escape of cystine from normal lysosomes follows saturation kinetics i.e., the rate increases linearly with increasing loading and levels off to a limiting value as the loading with cystine is further increased. The loaded preparations also exhibit the phenomenon of counter-transport i.e., the uptake of tracer amounts of cystine by the lysosomes is greatly stimulated by preloading of the lysosomes with unlabeled cystine; moveover, counter-transport was found to be inhibited by L-cystine but not by D-cystine. These observations are consistent with a stereo-specific carrier mechanism for transport of cystine in or out of normal leucocyte lysosomes. In contrast, similarly loaded lysosomes from cystinotic patients showed no egress of cystine nor did they demonstrate counter-transport, while lysosomes from obligate heterozygotes showed half the maximal velocities of egress or counter-transport associated with the normal systems. Results of this investigation indicate that human cystinosis is due to a defective carrier mechanism required for the escape of L-cystine from the intralysosomal space and provides an explanation for the massive intracellular accumulation of cystine seen in this disease. It is conceivable that other lysosomal storage disorders exist which are due to analogous defects in as yet undiscovered lysosomal carrier systems.